Continuously variable transmissions (CVTs) of the traction type are a common feature in engineering textbooks because their conceptual simplicity has a strong academic value. In practice, however, CVTs have many drawbacks which severely limit their applications. The typical CVT configurations (and the actual devices) include roll-torus (Matron Instruments, Inc.), planetary discs (Dodge Co.), planetary cones (Graham Company), spherical rollers (Cleveland Gear Company), and variable pulley with trapezoidal belt (Lewellen Manufacturing Corporation) according to Wilson, Sadler and Michels Kinematics and Dynamics of Machinery. Of all these configurations, only the variable pulley CVT has succeeded as a viable automobile application in the Subaru Justy (equipped with the smallest engine) in the USA.
The main disadvantage of CVT's is their low power capacity caused by power flow through a single point of contact. Attempts to increase the power paths to two or three have resulted in inexact control that tends to lock the device, lose power by slippage, and accelerate its wear. The variable pulley type has partially resolved this limitation, by replacing the single point of transfer by an arc of transfer, but for automobile applications it requires complex control, and a torque converter with its inherent loses.
One example of a known CVT is shown in U.S. Pat. No. 5,217,418 which pertains to a two-step toroidal CVT in which there are three points of power transfers per stage, each of which must be controlled separately. The transmission comprises two connected and oppositely facing discs and two sets of rollers to transmit drive between the discs. This type of transmission requires precise computer control which increases the expense of the device.
Another type of CVT is shown in U.S. Pat. No. 4,229,985. This transmission employs combinations of the following shapes: cones, internally tapered sleeves, dual faced internally tapered sleeves, and a combination of a dual faced internal wheel with a dual faced external shape. The primary purpose of the shapes and the manner in which they engage in traction is to achieve a matched geometry between the contacted rolling components, to increase the contacting surface areas, to engage each other in a point to point relationship, and to maintain a narrow or small difference between the largest diameter of any of the rolling components of the drive and the smallest diameter of any of the rolling components of the drive.
Many other CVT configurations have been suggested. However, none achieves the combination of high power capacity, efficiency, low cost and durability which is necessary for wide commercial acceptance.